kitty: allow float values in settings (#5925)

Some settings in kitty allow floating-point values, but this was not
reflected in the type.

tests/modules/programs/kitty/example-settings.nix

@@ -17,6 +17,7 @@ with lib;
         scrollback_lines = 10000;
         enable_audio_bell = false;
         update_check_interval = 0;
+        background_opacity = 0.5;
       };
 
       font.name = "DejaVu Sans";